U.S. patent number 6,992,614 [Application Number 10/829,849] was granted by the patent office on 2006-01-31 for radar altimeter.
This patent grant is currently assigned to Honeywell International Inc.. Invention is credited to James W. Joyce.
United States Patent |
6,992,614 |
Joyce |
January 31, 2006 |
**Please see images for:
( Certificate of Correction ) ** |
Radar altimeter
Abstract
A radar altimeter is provided that includes a transmitter
operable to generate a radio signal at a modulation frequency, and
transmit the radio signal toward a ground surface for reflection
therefrom to thereby propagate a reflected radio signal. The radar
altimeter also includes a receiver operable to receive the
reflected radio signal, and determine the altitude of the aircraft
based on the modulation frequency of the radio signal and a
difference frequency derived from the radio signal and the
reflected radio signal. The receiver is also operable to control
the transmitter so as to vary the modulation frequency of the radio
signal based on the altitude of the aircraft. Preferably, the
modulation frequency of the radio signal is greater at lower
altitudes than at higher altitudes.
Inventors: |
Joyce; James W. (Olathe,
KS) |
Assignee: |
Honeywell International Inc.
(Morristown, NJ)
|
Family
ID: |
35694864 |
Appl.
No.: |
10/829,849 |
Filed: |
April 22, 2004 |
Current U.S.
Class: |
342/122;
342/120 |
Current CPC
Class: |
G01S
7/352 (20130101); G01S 13/34 (20130101); G01S
13/882 (20130101) |
Current International
Class: |
G01S
13/32 (20060101) |
Field of
Search: |
;342/120,121,122,123 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Skolnik, Merrill I, Radar Handbook, pp. 16-28 to 16-32. cited by
other .
Technical Standard Order TSO-C87, Airborne Low-Range Radio
Altimeter; pp. 1-41, Feb. 1, 1966,
http://www.airweb.faa.gov/Regulatory.sub.--and.sub.--Guidance.sub.--Libra-
ry/rgTSO.nsf/0/2C417369F3ACFDD886256DC70062A5C1?OpenDocument. cited
by other .
RTCA D0-155, Minimum Performance Standards Airborne Low-Range
Altimeter, Nov. 1, 1974. cited by other.
|
Primary Examiner: Lobo; Ian J.
Claims
What is claimed and desired to be secured by Letters Patent is as
follows:
1. A radar altimeter for determining an altitude of an aircraft,
comprising: a first antenna; a second antenna; a transmitter
coupled to said first antenna and operable to generate a radio
signal at a modulation frequency and transmit said radio signal,
via said first antenna, toward a ground surface for reflection
therefrom to thereby propagate a reflected radio signal; and a
receiver coupled to said second antenna and configured to receive,
via said second antenna, said reflected radio signal and an antenna
coupling signal generated between said first and said second
antenna, the receiver operable to (i) determine said altitude of
said aircraft based on said modulation frequency of said radio
signal and a difference frequency derived from said radio signal
and said reflected radio signal and (ii) control said transmitter
so as to vary said modulation frequency of said radio signal based
on said altitude of said aircraft, said receiver comprising: a
mixer operable to (i) mix said radio signal with said reflected
radio signal and said antenna coupling signal to thereby generate a
mixed signal and (ii) demodulate said mixed signal into a baseband
difference signal and a baseband antenna coupling signal, and a
filter operable to attenuate said baseband antenna coupling signal
and pass said baseband difference signal without attenuation.
2. The radar altimeter of claim 1, wherein said radio signal
comprises a frequency modulated continuous wave radio signal.
3. The radar altimeter of claim 1, wherein said altitude of said
aircraft is determined based on said difference frequency derived
from said radio signal being transmitted at a specified time and
said reflected radio signal being received at said specified
time.
4. The radar altimeter of claim 1, wherein said modulation
frequency of said radio signal is varied when said altitude of said
aircraft reaches at least one threshold altitude.
5. The radar altimeter of claim 4, wherein said modulation
frequency of said radio signal is greater at altitudes below said
threshold altitude than at altitudes above said threshold
altitude.
6. The radar altimeter of claim 1, wherein said transmitter
comprises: a variable rate modulator operable to generate a voltage
waveform; and a voltage controlled oscillator operable to generate
said radio signal at said modulation frequency, wherein said
modulation frequency is controlled by said voltage waveform from
said variable rate modulator.
7. The radar altimeter of claim 6, wherein said receiver comprises
a microprocessor operable to control said variable rate modulator
so as to vary said modulation frequency of said radio signal based
on said altitude of said aircraft.
8. The radar altimeter of claim 1, wherein said filter comprises a
fixed filter having a single frequency response for all altitudes
of said aircraft.
9. The radar altimeter of claim 1, wherein said receiver further
comprises: an analog-to-digital converter operable to convert said
baseband difference signal to a digital difference signal; a
digital signal processor operable to determine said difference
frequency from said digital difference signal and correlate said
difference frequency to said altitude of said aircraft for said
modulation frequency; and a microprocessor operable to control said
transmitter so as to vary said modulation frequency of said radio
signal based on said altitude of said aircraft.
10. The radar altimeter of claim 9, wherein said microprocessor is
also operable to output said altitude of said aircraft to a display
of said altimeter.
11. A method of determining an altitude of an aircraft, comprising:
generating a radio signal at a modulation frequency; transmitting
said radio signal toward a ground surface for reflection therefrom
to thereby propagate a reflected radio signal; receiving said
reflected radio signal and, upon receipt thereof, generating an
antenna coupling signal; mixing said radio signal with said
reflected radio signal and said antenna coupling signal to thereby
generate a mixed signal; demodulating said mixed signal into a
baseband difference signal and a baseband antenna coupling signal;
attenuating said baseband antenna coupling signal and passing said
baseband difference signal without attenuation; determining said
altitude of said aircraft based on said modulation frequency of
said radio signal and a difference frequency derived from said
radio signal and said reflected radio signal; and varying said
modulation frequency of said radio signal based on said altitude of
said aircraft.
12. The method of claim 11, wherein said altitude of said aircraft
is determined based on said difference frequency derived from said
radio signal being transmitted at a specified time and said
reflected radio signal being received at said specified time.
13. The method of claim 11, wherein said modulation frequency of
said radio signal is varied when said altitude of said aircraft
reaches at least one threshold altitude.
14. The method of claim 13, wherein said modulation frequency of
said radio signal is greater at altitudes below said threshold
altitude than at altitudes above said threshold altitude.
15. The method of claim 11, further comprising: generating a
voltage waveform to control said modulation frequency of said radio
signal; generating said radio signal at said modulation frequency
controlled by said voltage waveform; and varying said voltage
waveform based on said altitude of said aircraft so as to vary said
modulation fluency of said radio signal.
16. The method of claim 11, wherein said attenuating is performed
by a fixed filter having a single frequency response for all
altitudes of said aircraft.
17. The method of claim 11, wherein said method further comprises:
converting said baseband difference signal to a digital difference
signal; determining said difference frequency from said digital
difference signal; correlating said difference frequency to said
altitude of said aircraft for said modulation frequency; and
varying said modulation frequency based on said altitude of said
aircraft.
18. A radar altimeter for determining an altitude of an aircraft,
comprising: means for generating a radio signal at a modulation
frequency; means for transmitting said radio signal toward a ground
surface for reflection therefrom to thereby propagate a reflected
radio signal; means for receiving said reflected radio signal and
an antenna coupling signal; means for (i) mixing said radio signal
with said reflected radio signal and said antenna coupling signal
to thereby generate a mixed signal and (ii) demodulating said mixed
signal into a baseband difference signal and a baseband antenna
coupling signal, means for (i) attenuating said baseband antenna
coupling signal and (ii) passing said baseband difference signal
without attenuation; means for determining said altitude of said
aircraft based on said modulation frequency of said radio signal
and a difference frequency derived from said radio signal and said
reflected radio signal; and means for varying said modulation
frequency of said radio signal based on said altitude of said
aircraft.
19. The radar altimeter of claim 18, wherein said radio signal
comprises a frequency modulated continuous wave radio signal.
20. The radar altimeter of claim 18, wherein said altitude of said
aircraft is determined based on said difference frequency derived
from said radio signal being transmitted at a specified time and
said reflected radio signal being received at said specified
time.
21. The radar altimeter of claim 18, wherein said modulation
frequency of said radio signal is varied when said altitude of said
aircraft reaches at least one threshold altitude.
22. The radar altimeter of claim 21, wherein said modulation
frequency of said radio signal is greater at altitudes below said
threshold altitude than at altitudes above said threshold
altitude.
23. A radar altimeter for determining an altitude of an aircraft,
comprising: a variable rate modulator operable to generate a
voltage waveform; a voltage controlled oscillator operable to
generate a frequency modulated continuous wave radio signal at a
modulation frequency controlled by said voltage waveform from said
variable rate modulator; a transmitter antenna operable to transmit
said radio signal toward a ground surface for reflection therefrom
to thereby propagate a reflected radio signal; a receiver antenna
operable to receive said reflected radio signal, wherein said
receiver antenna also receives an antenna coupling signal generated
between said transmitter antenna and said receiver antenna; a mixer
operable to mix said radio signal with said reflected radio signal
and said antenna coupling signal to thereby generate a mixed
signal, said mixer also operable to demodulate said mixed signal
into a baseband difference signal and a baseband antenna coupling
signal; a filter operable to attenuate said baseband antenna
coupling signal and pass said baseband difference signal without
attenuation; an analog-to-digital converter operable to convert
said baseband difference signal to a digital difference signal; a
digital signal processor operable to determine a difference
frequency from said digital difference signal and correlate said
difference frequency to said altitude of said aircraft for said
modulation frequency; and a microprocessor operable to control said
variable rate modulator so as to vary said modulation frequency of
said radio signal based on said altitude of said aircraft.
24. The radar altimeter of claim 23, wherein said altitude of said
aircraft is determined based on said difference frequency derived
from said radio signal being transmitted at a specified time and
said reflected radio signal being received at said specified
time.
25. The radar altimeter of claim 23, wherein said modulation
frequency of said radio signal is varied when said altitude of said
aircraft reaches at least one threshold altitude.
26. The altimeter of claim 25, wherein said modulation frequency of
said radio signal is greater at altitudes below said threshold
altitude than at altitudes above said threshold altitude.
27. The radar altimeter of claim 23, wherein said filter comprises
a fixed filter having a single frequency response for all altitudes
of said aircraft.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to radar altimeters, and
more particularly to frequency modulated continuous wave (FMCW)
radar altimeters used for aviation navigation.
2. Description of Related Art
Frequency modulated continuous wave (FMCW) radar altimeters are
used by pilots to determine the altitude of an aircraft in
flight-critical situations, such as making an instrument landing in
low visibility conditions. A FMCW radar altimeter generally
comprises a transmitter that transmits a radio signal toward the
ground surface, and a receiver that receives the radio signal after
it has reflected from the ground surface. The receiver mixes the
transmitted radio signal with the received reflected radio signal
and thereby generates a difference signal. The receiver uses the
frequency of this difference signal to determine the altitude of
the aircraft (wherein the frequency is proportional to the
altitude). This altitude measurement is then output to a radar
altimeter display located within the cockpit of the aircraft.
One inherent problem with the design of a FMCW radar altimeter is
that there will be a certain amount of coupling from the
transmitter antenna to the receiver antenna. This antenna coupling
is particularly problematic at higher altitudes where the magnitude
of the antenna coupling signal is significant compared to the
magnitude of the received reflected radio signal. As such, the
receiver will occasionally lock on to the antenna coupling signal,
and then erroneously use the frequency of the antenna coupling
signal to determine the altitude of the aircraft. When this
happens, the needle of the radar altimeter display drops to
approximately 0 feet, resulting in undue pilot concern or even the
loss of the pilot's confidence in the radar altimeter.
An attempt to solve this problem has been to utilize a switched
filter in the receiver of a FMCW radar altimeter for the purpose of
attenuating the antenna coupling signal at higher altitudes. The
switched filter is designed to have one frequency response at lower
altitudes (which passes both the difference signal and the antenna
coupling signal) and another frequency response at higher altitudes
(which passes the difference signal and attenuates the antenna
coupling signal). Thus, in operation, the receiver will properly
lock on to the difference signal both at lower altitudes (where the
magnitude of the difference signal is relatively large compared to
the magnitude of the antenna coupling signal) and at higher
altitudes (where the antenna coupling signal has been
attenuated).
There are several disadvantages, however, associated with the use
of a switched filter within the receiver of a FMCW radar altimeter.
For example, because the characteristics of a switched filter are
fixed, various hardware components of the receiver must be changed
in order to modify the filter parameters. As such, the switched
filter may not be customized on an individual installation basis.
Thus, there is a need for a FMCW radar altimeter that does not use
a switched filter to attenuate the antenna coupling signal.
SUMMARY OF THE INVENTION
The present invention is directed to a FMCW radar altimeter that
generally comprises a transmitter and a receiver. The transmitter
is operable to generate a radio signal at a specified modulation
frequency, and transmit the radio signal toward the ground surface
for reflection therefrom to thereby propagate a reflected radio
signal. The receiver is operable to receive the reflected radio
signal from the ground surface, and determine the altitude of the
aircraft based on two different factors: (1) the modulation
frequency of the radio signal; and (2) a difference frequency
derived from the radio signal and the reflected radio signal. The
receiver is also operable to control the transmitter so as to vary
the modulation frequency of the radio signal based on the altitude
of the aircraft. Preferably, the modulation frequency of the radio
signal is greater at lower altitudes than at higher altitudes.
In an exemplary embodiment, the transmitter includes a variable
rate modulator that generates a voltage waveform. The transmitter
also includes a voltage controlled oscillator that generates a
radio signal at a specified modulation frequency, which is
controlled by the voltage waveform from the variable rate
modulator. Also included is a transmitter antenna that transmits
the radio signal toward the ground surface for reflection therefrom
to thereby propagate a reflected radio signal.
The receiver includes a receiver antenna that receives the
reflected radio signal from the ground surface, and also detects an
unwanted antenna coupling signal from the transmitter antenna. A
mixer is provided that mixes the radio signal, the reflected radio
signal, and the unwanted antenna coupling signal and thereby
generates a mixed signal. The mixer then demodulates the mixed
signal into a baseband difference signal (having a difference
frequency derived from the radio signal and the reflected radio
signal) and a baseband antenna coupling signal. The receiver also
includes a fixed filter designed to attenuate the baseband antenna
coupling signal and pass the baseband difference signal.
Significantly, the fixed filter has a single frequency response for
all altitudes of the aircraft such that a switched filter is not
required.
The receiver further includes an analog-to-digital converter that
converts the baseband difference signal to a digital difference
signal. A digital signal processor is also provided that determines
the difference frequency from the digital difference signal, and
then correlates the difference frequency to the altitude of the
aircraft for the particular modulation frequency of the radio
signal. The receiver also includes a microprocessor that controls
the variable rate modulator of the transmitter so as to vary the
modulation frequency of the radio signal based on the altitude of
the aircraft. Preferably, the modulation frequency of the radio
signal is varied when the altitude of the aircraft reaches one or
more threshold altitudes, such that the modulation frequency is
greater at altitudes below a particular threshold altitude than at
altitudes above the particular threshold altitude. By varying the
modulation frequency of the radio signal, the receiver is able to
obtain more accurate altitude measurements when the aircraft is
near the ground surface.
The present invention will be better understood from the following
detailed description of the invention, read in connection with the
drawings as hereinafter described.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a radar altimeter in accordance with
an exemplary embodiment of the present invention, showing the
various functional blocks of the transmitter and the receiver.
FIG. 2 is a graphical representation of the output of the mixer of
the receiver shown in FIG. 1, wherein the radio signal has been
modulated at the nominal modulation frequency.
FIG. 3 is a graphical representation of the output of the mixer of
the receiver shown in FIG. 1, wherein the radio signal has been
modulated at twice the nominal modulation frequency.
FIG. 4 is a graphical representation of the output of the mixer of
the receiver shown in FIG. 1, wherein the radio signal has been
modulated at four times the nominal modulation frequency.
FIG. 5 is a graphical representation of the frequency response of
the fixed filter of the receiver shown in FIG. 1, showing
attenuation of the baseband antenna coupling signal when the radio
signal has been modulated at the nominal modulation frequency,
twice the nominal modulation frequency, and four times the nominal
modulation frequency.
DETAILED DESCRIPTION OF THE INVENTION
A frequency modulated continuous wave (FMCW) radar altimeter in
accordance with an exemplary embodiment of the present invention is
depicted in FIG. 1 (with graphical representations of the
functionality of the radar altimeter depicted in FIGS. 2 5). While
the invention will be described in detail hereinbelow with
reference to this exemplary embodiment, it should be understood
that the invention is not limited to the specific architecture of
the radar altimeter shown in this embodiment. Rather, one skilled
in the art will appreciate that a wide variety of radar altimeter
architectures may be implemented in accordance with the present
invention.
Referring to FIG. 1, a radar altimeter in accordance with an
exemplary embodiment of the present invention includes a
transmitter 10 (which generally comprises a variable rate modulator
12, a voltage controlled oscillator 14, a transmitter amplifier 16,
and a transmitter antenna 18) and a receiver 20 (which generally
comprises a receiver antenna 22, a receiver amplifier 24, a mixer
26, a mixed signal amplifier 28, a fixed filter 30, an
analog-to-digital converter 32, a digital signal processor 34, and
a microprocessor 36). Preferably, the radar altimeter is designed
to comply with various aviation industry specifications known in
the art, such as TSO C87, RTCA DO-178B and RTCA DO-160D.
As will be described in greater detail hereinbelow, transmitter 10
is operable to generate a radio signal at a specified modulation
frequency, and transmit the radio signal toward the ground surface
for reflection therefrom to thereby propagate a reflected radio
signal. Receiver 20 is operable to receive the reflected radio
signal from the ground surface, and determine the altitude of the
aircraft based on two different factors: (1) the modulation
frequency of the radio signal; and (2) a difference frequency
derived from the radio signal and the reflected radio signal.
Receiver 20 is also operable to control transmitter 10 so as to
vary the modulation frequency of the radio signal based on the
altitude of the aircraft, wherein the modulation frequency is
greater at lower altitudes than at higher altitudes. As will be
seen, by varying the modulation frequency of the radio signal based
on the altitude of the aircraft, receiver 20 is able to utilize
fixed filter 30 which has a single frequency response for all
altitudes of the aircraft. In addition, receiver 20 is able to
obtain more accurate altitude measurements when the aircraft is
near the ground surface.
Looking more closely to transmitter 10 in FIG. 1, variable rate
modulator 12 generally operates as a driver circuit for voltage
controlled oscillator 14. In particular, variable rate modulator 12
generates a voltage waveform that causes voltage controlled
oscillator 14 to frequency modulate and thereby generate a FMCW
radio signal. As such, the voltage waveform generated by variable
rate modulator 12 controls the modulation frequency of the radio
signal. As will be described in greater detail hereinbelow,
microprocessor 36 of receiver 20 controls variable rate modulator
12 so as to change the voltage waveform generated by variable rate
modulator 12 when the altitude of the aircraft reaches one or more
predetermined threshold altitudes, thereby changing the modulation
frequency of the radio signal.
Voltage controlled oscillator 14 is operable to receive the voltage
waveform from variable rate modulator 12 and frequency modulate an
RF waveform of constant amplitude to generate a FMCW radio signal.
Typically, the center frequency of the FMCW radio signal is set to
4.3 GHz, although any center frequency may be used. The period of
modulation may vary from 0.01 .mu.s (corresponding to a modulation
frequency of 100 Hz) to 0.0095 .mu.s (corresponding to a modulation
frequency of 105 Hz), although any period of modulation and
corresponding modulation frequency may be used. Preferably, the
radio signal is modulated such that the frequency increases and
decreases linearly as it varies in time. As is known in the art,
the slope of the radio signal is the frequency deviation rate and
is typically expressed in hertz per foot (Hz/ft) of altitude. As
will be described in greater detail hereinbelow, digital signal
processor 34 of receiver 20 is able to use the modulation frequency
and corresponding frequency deviation rate (in conjunction with a
difference frequency described below) to provide an accurate
measurement of the altitude of the aircraft above the ground
surface.
Transmitter amplifier 16 is operable to receive the radio signal
from voltage controlled oscillator 14 and increase the amplitude of
the radio signal before it is transmitted through transmitter
antenna 18. Preferably, the output power of transmitter amplifier
16 is sufficient to ensure that the radio signal may be detected by
receiver antenna 22 after reflection from the ground surface, which
is particularly critical at higher altitudes of the aircraft. Thus,
transmitter 10 may be "matched" to receiver 20 such that a
transmitter having a lower output power may be used in connection
with a receiver having better detection capabilities, and
vice-versa. A typical output power for transmitter amplifier 16 is
160 mW.
Transmitter antenna 18 is operable to receive the radio signal from
transmitter amplifier 16 and transmit the radio signal toward the
ground surface. It should be understood that the radio signal then
reflects off the ground surface to thereby propagate a reflected
radio signal. Receiver antenna 22 is then operable to detect and
receive the reflected radio signal propagated from the ground
surface.
As is known in the art, transmitter antenna 18 and receiver antenna
22 are preferably mounted at least 20 inches apart near the point
of aircraft rotation and as close as feasible to the
receiver/transmitter box. It is also preferable to mount both
antennas such that they are not located near other antennas or
aircraft projections (including landing gear, flaps, etc.).
Preferably, both antennas are mounted such that they point straight
downward (e.g., within 6 degrees) when the aircraft is in level
flight.
Because transmitter antenna 18 and receiver antenna 22 are both
located on the same aircraft, it is known in the art that an
unwanted antenna coupling signal will be generated from transmitter
antenna 18 to receiver antenna 22. As such, receiver antenna 22
will detect and receive both the reflected radio signal and the
unwanted antenna coupling signal. As will be described below, the
unwanted antenna coupling signal is attenuated by fixed filter 30
in accordance with the present invention.
Receiver amplifier 24 is operable to receive the reflected radio
signal from receiver antenna 22 and increase the amplitude of the
reflected radio signal so that it may be easily processed by
subsequent circuitry within receiver 20. Preferably, the output
power of receiver amplifier 24 is great enough to meet the input
requirements of mixer 26, even at higher altitudes where the power
of the reflected radio signal is weaker. Of course, receiver
amplifier 24 will also increase the amplitude of the unwanted
antenna coupling signal received from receiver antenna 22.
Mixer 26 is operable to receive the reflected radio signal and the
unwanted antenna coupling signal from receiver amplifier 24, and is
also connected to transmitter 10 so as to receive the radio signal
from voltage controlled oscillator 14 prior to transmission.
Preferably, mixer 26 receives the radio signal being transmitted at
a specified time and the reflected radio being received at that
same specified time. Mixer 26 is then operable to mix the radio
signal with the reflected radio signal and the unwanted antenna
coupling signal, and then demodulate these signals so as to
generate a baseband difference signal and a baseband antenna
coupling signal. The frequency of the baseband difference signal
(hereinafter referred to as the "difference frequency") is derived
from the difference between the frequencies of the transmitted
radio signal and the received reflected radio signal. One skilled
in the art will understand that the difference frequency is
proportional to the altitude of the aircraft, and ranges from
f.sub.min (corresponding to the altitude of the aircraft on the
ground surface) to f.sub.max (corresponding to the altitude of the
aircraft at the maximum of the altimatic scale, commonly 2,500 feet
above the ground surface).
FIGS. 2 4 are graphical representations of the output of mixer 26,
wherein the radio signal has been modulated at three different
modulation frequencies, namely, a nominal modulation frequency
(1.times.) of 100 Hz (see FIG. 2), twice the nominal modulation
frequency (2.times.) of 200 Hz (see FIG. 3), and four times the
nominal modulation frequency (4.times.) of 400 Hz (see FIG. 4).
Each of these graphical representations show the approximate
frequencies and magnitudes of the baseband antenna coupling signal
and the baseband difference signal for various altitudes of the
aircraft. It should be understood that the magnitudes of the
various signals are not to scale and are intended only to show the
relative strength between the signals. Similarly, the frequencies
of the various signals are approximated and are intended only to
show the relative frequencies between the signals. It should
further be understood that the modulation frequencies shown in the
graphical representations of FIGS. 2 4 are merely examples and that
a plurality of other modulation frequencies could be used in
accordance with the present invention (e.g., 1.5 times the nominal
modulation frequency, 5 times the nominal modulation frequency,
etc.).
In FIG. 2, the radio signal has been modulated at a nominal
modulation frequency (1.times.) of 100 Hz, which corresponds to a
frequency deviation rate of 40 Hz/ft. As can be seen, the frequency
of the baseband antenna coupling signal is approximately 800 Hz. It
should be understood that the exact frequency of the baseband
antenna coupling signal may vary from installation to installation
and is dependent on a number of variables, including antenna
spacing and the length of the antenna cables. It can also be seen
that the frequency of the baseband difference signal is
approximately 20 kHz at an altitude of 500 feet and approximately
100 kHz at an altitude of 2,500 feet. Furthermore, it can be seen
that the magnitude of the baseband antenna coupling signal is
significantly less than the magnitude of the baseband difference
signal at an altitude of 500 feet, and is also less than the
magnitude of the baseband difference signal at an altitude of 2,500
feet.
In FIG. 3, the radio signal has been modulated at twice the nominal
modulation frequency (2.times.) of 200 Hz, which corresponds to a
frequency deviation rate of 80 Hz/ft. As can be seen, the frequency
of the baseband antenna coupling signal is approximately 1.6 kHz.
Again, it should be understood that the exact frequency of the
baseband antenna coupling signal may vary from installation to
installation and is dependent on a number of variables, including
antenna spacing and the length of the antenna cables. It can also
be seen that the frequency of the baseband difference signal is
approximately 20 kHz at an altitude of 250 feet and approximately
100 kHz at an altitude of 1,250 feet. Furthermore, it can be seen
that the magnitude of the baseband antenna coupling signal is
significantly less than the magnitude of the baseband difference
signal at an altitude of 250 feet, and is also less than the
magnitude of the baseband difference signal at an altitude of 1,250
feet.
In FIG. 4, the radio signal has been modulated at four times the
nominal modulation frequency (4.times.) of 400 Hz, which
corresponds to a frequency deviation rate of 160 Hz/ft. As can be
seen, the frequency of the baseband antenna coupling signal is
approximately 3.2 kHz. Yet again, it should be understood that the
exact frequency of the baseband antenna coupling signal may vary
from installation to installation and is dependent on a number of
variables, including antenna spacing and the length of the antenna
cables. It can also be seen that the frequency of the baseband
difference signal is approximately 20 kHz at an altitude of 125
feet and approximately 100 kHz at an altitude of 625 feet.
Furthermore, it can be seen that the magnitude of the baseband
antenna coupling signal is significantly less than the magnitude of
the baseband difference signal at an altitude of 125 feet, and is
also less than the magnitude of the baseband difference signal at
an altitude of 625 feet.
Several observations can be made from the graphical representations
of FIGS. 2 4. First, for any of the modulation frequencies, the
frequency of the baseband difference signal is proportional to the
altitude of the aircraft (i.e., the frequency increases as the
altitude increases) and the magnitude of the baseband difference
signal is inversely proportional to the altitude of the aircraft
(i.e., the magnitude decreases as the altitude increases). Second,
the frequency of the baseband difference signal is proportional to
the modulation frequency of the radio signal (i.e., the frequency
increases as the modulation frequency increases).
Third, the baseband difference signal will have approximately the
same frequency for different combinations of altitude and
modulation frequency. For example, the difference frequency is 20
kHz for: an aircraft flying at 500 feet with a modulation frequency
of 100 Hz (see FIG. 2); an aircraft flying at 250 feet with a
modulation frequency of 200 Hz (see FIG. 3); and an aircraft flying
at 125 feet with a modulation frequency of 400 Hz (see FIG. 4).
Similarly, the difference frequency is 100 kHz for: an aircraft
flying at 2,500 feet with a modulation frequency of 100 Hz (see
FIG. 2); an aircraft flying at 1,250 feet with a modulation
frequency of 200 Hz (see FIG. 3); and an aircraft flying at 625
feet with a modulation frequency of 400 Hz (see FIG. 4).
As will be described in greater detail hereinbelow, the modulation
frequency of the radio signal may be varied when the aircraft
reaches one or more predetermined threshold altitudes. In general,
the modulation frequency of the radio signal is greater at lower
altitudes than at higher altitudes. Using a greater modulation
frequency at lower altitudes causes the difference frequency of the
baseband difference signal to be increased and shifted away from
the frequency of the baseband antenna coupling signal. For example,
looking to FIG. 4, it can be appreciated that an aircraft flying at
500 feet with a radio signal generated at four times the nominal
modulation frequency (4.times.) of 400 Hz will produce a baseband
difference signal having a difference frequency of approximately 80
kHz (as opposed to 20 kHz if the radio signal had been generated at
the nominal modulation frequency (1.times.) of 100 Hz, as shown in
FIG. 2). It will be seen that varying the modulation frequency at
one or more predetermined threshold altitudes enables the use of
fixed filter 30 (described below) for all altitudes of the aircraft
such that a switched filter is not required.
Referring again to FIG. 1, mixed signal amplifier 28 is operable to
receive the baseband difference signal from mixer 26 and increase
the amplitude of the baseband difference signal. Preferably, the
output power of mixed signal amplifier 28 is sufficient to ensure
that the baseband difference signal may be easily processed by
downstream circuits within receiver 20. Of course, mixed signal
amplifier 28 will also increase the amplitude of the unwanted
baseband antenna coupling signal received from mixer 26.
Fixed filter 30 is operable to receive the baseband difference
signal and the baseband antenna coupling signal from mixed signal
amplifier 28, and filter the unwanted baseband antenna coupling
signal therefrom. A graphical representation of an exemplary
frequency response of fixed filter 30 is shown in FIG. 5. In this
example, fixed filter 30 comprises a band-pass filter that is tuned
to pass signals within the 10 kHz to 100 kHz frequency range
without attenuation. It can also be seen that the band-pass filter
will pass the signals between 4-kHz and 10 kHz (with varying
degrees of attenuation) and the signals between 100 kHz and 300 kHz
(with varying degrees of attenuation). In addition, the band-pass
filter will significantly attenuate all signals below 4 kHz and
above 300 kHz. As such, in this example, the band-pass filter will
pass any of the baseband difference signals shown in FIGS. 2 4 (all
of which fall within the 10 kHz to 100 kHz frequency range) and
attenuate any of the baseband antenna coupling signals shown in
FIGS. 2 4 (all of which fall below 4 kHz).
Looking at FIGS. 2 4 in conjunction with FIG. 5, it can be seen
that increasing the modulation frequency of the radio signal at
lower altitudes causes the baseband difference signals to increase
in frequency and shift within the passband of fixed filter 30. It
should be understood that if the modulation frequency of the radio
signal was not increased at lower altitudes, the baseband
difference signals would fall below the passband of fixed filter 30
and would thus be attenuated (such that it would be necessary to
use a switched filter having one frequency response at lower
altitudes and another frequency response at higher altitudes).
Thus, in accordance with the present invention, varying the
modulation frequency of the radio signal at one or more
predetermined threshold altitudes enables the use of fixed filter
30 having the same frequency response for all altitudes of the
aircraft.
Referring again to FIG. 1, analog-to-digital converter 32 is
operable to receive the baseband difference signal from fixed
filter 30 and convert the baseband difference signal to a digital
difference signal. Preferably, the sampling rate, resolution and
spurious-free dynamic range of analog-to-digital converter 32 is
selected to ensure that the converted signals comply with aviation
industry specifications. Of course, it should be understood that
any analog-to-digital converter that meets the conversion
requirements for a particular application may be used.
Digital signal processor 34 is operable to receive the digital
difference signal from analog-to-digital converter 32 and determine
the difference frequency therefrom (which, as discussed above, is
derived from the difference between the frequencies of the
transmitted radio signal and the received reflected radio signal).
Digital signal processor 34 is then operable to correlate the
difference frequency to the altitude of the aircraft for the
particular modulation frequency of the radio signal. In other
words, digital signal processor 34 determines the altitude of the
aircraft based on two different factors: (1) the modulation
frequency of the radio signal; and (2) the difference frequency
extracted from the digital difference signal.
For example, looking to FIG. 2, a radio signal modulated at the
nominal modulation frequency (1.times.) of 100 Hz has a
corresponding frequency deviation rate of 40 Hz/ft. At this
modulation frequency, if the difference frequency extracted from
the digital difference signal were 20 kHz, it follows that the
altitude of the aircraft is 500 feet (i.e., 20 kHz divided by 40
Hz/ft).
As another example, looking to FIG. 3, a radio signal modulated at
twice the nominal modulation frequency (2.times.) of 200 Hz has a
corresponding frequency deviation rate of 80 Hz/ft. At this
modulation frequency, if the difference frequency extracted from
the digital difference signal were 20 kHz, it follows that the
altitude of the aircraft is 250 feet (i.e., 20 kHz divided by 80
Hz/ft).
As yet another example, looking to FIG. 4, a radio signal modulated
at four times the nominal modulation frequency (4.times.) of 400 Hz
has a corresponding frequency deviation rate of 160 Hz/ft. At this
modulation frequency, if the difference frequency extracted from
the digital difference signal were 20 kHz, it follows that the
altitude of the aircraft is 125 feet (i.e., 20 kHz divided by 160
Hz/ft).
In all three examples, it should be noted that the difference
frequency extracted from the digital difference signal is 20 kHz.
Thus, it is necessary to know both the difference frequency and the
modulation frequency and corresponding frequency deviation rate of
the radio signal in order to determine the altitude of the
aircraft.
Referring yet again to FIG. 1, microprocessor 36 is operable to
receive the altitude measurement from digital signal processor 34
and generate an output signal that is transmitted to the radar
altimeter display located within the cockpit of the aircraft.
Microprocessor 36 is also operable to control variable rate
modulator 12 of transmitter 10 so as to vary the modulation
frequency of the radio signal based on the altitude of the
aircraft. Preferably, the modulation frequency of the radio signal
is varied when the altitude of the aircraft reaches one or more
predetermined threshold altitudes, such that the modulation
frequency is greater at altitudes below a particular threshold
altitude than at altitudes above the particular threshold altitude.
Microprocessor 36 also transmits the modulation frequency to
digital signal processor 34 for use in determining the altitude of
the aircraft (described above). Of course, it should be understood
that all of the various functions of microprocessor 36 could be
incorporated into digital signal processor 34.
An example will now be provided in which the modulation frequency
of the radio signal is varied at two predetermined threshold
altitudes in accordance with the present invention. Looking to
FIGS. 2 4, the predetermined threshold altitudes may comprise 1,000
feet and 500 feet. In this example, when the aircraft is flying
above 1,000 feet, the radio signal would be generated at the
nominal modulation frequency (1.times.) of 100 Hz. When the
aircraft drops below the first threshold altitude of 1,000 feet,
the radio signal would be generated at twice the nominal modulation
frequency (2.times.) of 200 Hz. Then, when the aircraft drops below
the second threshold altitude of 500 feet, the radio signal would
be generated at four times the nominal modulation frequency
(4.times.) of 400 Hz. Of course, it should be understood that any
number of predetermined threshold altitudes may be utilized in
accordance with the present invention.
It can be appreciated that increasing the modulation frequency of
the radio signal at lower altitudes enables the radar altimeter to
obtain more accurate altitude measurements when the aircraft is
near the ground surface. Specifically, when using a higher
modulation frequency, a given change in altitude results in a
larger change in difference frequency. For example, in FIG. 4, a
change in altitude of 500 feet (e.g., 125 feet to 625 feet) would
result in a change in difference frequency of 80 kHz (e.g., 20 kHz
to 100 kHz). By contrast, in FIG. 3, a change in altitude of 500
feet (e.g., 250 feet to 750 feet) would result in a change in
difference frequency of 40 kHz (e.g., 20 kHz to 60 kHz). Thus, at
higher modulation frequencies, the increased resolution results in
more accurate altitude measurements for lower altitudes of the
aircraft.
While the present invention has been described and illustrated
hereinabove with reference to an exemplary embodiment, it should be
understood that various modifications could be made to this
embodiment without departing from the scope of the invention.
Therefore, the invention is not to be limited to the specific
embodiment described and illustrated hereinabove, except insofar as
such limitations are included in the following claims.
* * * * *
References